There are many methods for making thermoplastic or stampable sheet products from reinforcing fibres and polymers. Extrusion of fibres and plastics into a sheet is one common approach. Subsequent compression moulding or thermoforming can then take place to produce consolidated sheets or moulded shapes using matched die moulds. Layered arrangements of polymer films and fibre mats (textile/woven or non-woven) which are subsequently pressed is another route (film stacking or lamination). There are a number of variations around these processes for continuous or semi-continuous or automated processes. Glass fibres are commonly used but others are also used. A relatively common product is GMT (glass mat thermoplastic) composite. GMT composites can be moulded and stamped into a variety of suitable products, for example automotive bumpers. Thus, processes for preparing thermo-mouldable, thermoplastic composite material are already known in the art. For instance, U.S. Pat. No. 949,133 describes processes for preparing slabs from thermo-mouldable composite material obtained by inserting reinforcing fibre layers between the sheets of previously extruded polymer. The panel thus obtained is compressed under heat in a manner such as to allow the molten polymer to penetrate the reinforcing structure.
The reinforcements generally comprise glass fibers and consists of small mats of chopped fibers or of small, mono-yarn mats of continuous fibers. Other processes have been suggested such as, for example, those described in GB-A-1 010 043 or U.S. Pat. No. 3,396,142. According to these processes the powdered thermoplastic matrix is fed after the small fibrous mats have been prepared.
EP0235794B1 describes a process which comprises simultaneously and separately feeding the granulated thermoplastic polymer and a reinforcement selected from natural, artificial or synthetic fibers to a conveyor belt and embedding and heating under pressure the structure thus obtained. A beating operation is included for the purpose of enhancing the penetration of the polymer into the reinforcing structure but it also serves to disperse, as much as possible, the multi-filament forming the reinforcing structure into single mono-filaments.
EP0312090B1 describes a process of producing a fibrous laminate which comprises laminating a thermoplastic resin fibre binder non-woven fabric (A) having a melting point of from 60 to 200C. on the surface of a fiber mat (B) composed of synthetic or natural fibres, said synthetic fibers or natural fibers having a melting point of at least 40C higher than the melting point of the aforesaid fibre binder non-woven fabric, applying needling to the laminate from the side of the thermoplastic resin fibre binder non-woven fabric (A), whereby the binder fibres of the non-woven fabric (A) and the fibres of the fibre mat (B) are entangled with each other to provide a laminate mat (C), placing a surface decorating material (D) on the surface of the non-woven fabric (A), and heating the laminate to a temperature of not melting the surface decorating material (D) and the fibres of the fibre mat (B) but melting the binder fibres of the non-woven fabric (A) to weld the surface decorating material (D) and the laminate mat (C) in a body.
Della Vecchia et al, in U.S. Pat. No. 4,612,238, discloses a process for producing a laminate that combines a layer of resin, a glass mat, another layer of resin, a second glass mat, and a third layer of resin. The first and third layers of resin, which are thermoplastic materials, can be so selected to obtain desired properties. Various quantities of reinforcing materials, fillers and additives can be included in selected proportions.
There is increasing interest in wood and natural fibre utilisation in plastics. Primarily this has been focussed on extrusion based processing, though sheet pressing or thermoforming methods have also been reported and developed. A J Mitchell (1990; Forestry & forest Products, Newsletter, Vol 4, No 2, Autumn CSIRO, Australia, pp 1-4 describes common approaches to wood fibre plastic sheet manufacture. Examples described include dry formed wood fibre sheets with plastics, wood fibre sheets prepared by organic solvent based impregnation, wood fibre sheets impregnated by dipping with emulsions in water, and molten polymer impregnated sheets and yarns. U.S. Pat. No. 4,562,218 describes formable pulp compositions from aqueous wood and natural fibre slurries.
Drawbacks of the above-mentioned processes often include poor economy in that the reinforcement may be relatively expensive or expensively produced and its mat or web has to be prepared separately from the fibre production, or a separate drying process or impregnation is required, or the methods of combination of wood or natural fibres with the polymer matrix/compositions are inconvenient and there are difficulties in handling of short or low bulk density fibres.
Plastic and wood, or plastic and natural fibres, do not mix easily, although some polymers are more compatible than others are. PVC, which is polar like wood, reportedly bonds to, or interacts well with, the filler or fibre without special alloying or coupling agents, whereas polyolefins (polypropylene and polyethylene) do not adhere to wood as well as PVC, and so such wood-plastics requite modification to get the best level of performance from the filler or fibre in the plastic. One example of the current state of technology is to add a coupling agent, often a maleated polyolefin for polyolefin based composites, into the extruder and mixing within the extruder. The prior art contains numerous suggestions regarding polymer fibre composites. Gaylord, U.S. Pat. Nos. 3,765,934, 3,869,432, 3,894,975, 3,900,685, 3,958,069 and Casper et al., U.S. Pat. No. 4,051,214 teach a bunk polymerization that occurs in situ between styrene and maleic anhydride monomer combined with wood fibre to prepare a polymer fibre composite. Segaud, U.S. Pat. No. 4,528,303 teaches a composite composition containing a polymer, a reinforcing mineral filler and a coupling agent that increases the compatibility between the fillet and the polymer. The prior art also recognizes modifying the fibre component of a composite. Hamed, U.S. Pat. No. 3,943,079 teaches subjecting unregenerated discontinuous cellulose fibre to a shearing force in shear mixers, resulting in mixing of a polymer and a lubricant material with the fibre. Such processing improves fibre separation and prevents agglomeration. Similarly, Coran et al., U.S. Pat. No. 4,414,267 teaches a treatment of fibre with an aqueous dispersion of a vinyl chloride polymer and a plasticizer, the resulting fibres contain a coating of polyvinyl chloride and plasticizer and can be incorporated into the polymer matrix with reduced mixing energy. Beshay, U.S. Pat. Nos. 4,717,742 and 4,820,749 teach a composite material containing a cellulose having grafted silane groups. Raj et al., U.S. Pat. No. 5,120,776 teach cellulosic fibres pre-treated with maleic or phthalic anhydride to improve the bonding and dispersibility of the fibre in the polymer matrix. Raj et al. teach a high density polyethylene chemical treated pulp composite. Hon, U.S. Pat. No. 5,288,772 discloses fibre reinforced thermoplastic made with a moisture pre-treated cellulosic material such as discarded newspapers having a lignin content. Kokta et al., “Composites of Poly(Vinyl Chloride) and Wood Fibres. Part II. Effect of Chemical Treatment”, Polymer Composites, April 1990, Volume 11, No. 2, teach a variety of cellulose treatments. The treatments include latex coating, grafting with vinyl monomers, grafting with acids or anhydrides, grafting with coupling agents such as maleic anhydride, abietic acid (See also Kokta, U.K. Application No. 2,192,397). Beshay, U.S. Pat. No. 5,153,241 teaches composite materials including a modified cellulose. The cellulose is modified with an organo titanium coupling agent which reacts with and reinforces the polymer phase. Maldas and Kokta, “Surface modification of wood fibres using maleic anhydride and isocyanate as coating components and their performance in polystyrene composites”, Journal Adhesion Science Technology, 1991, pp. 1-14 show polystyrene flour composites containing a maleic anhydride modified wood flour. A number of publications including Kokta et al., “Composites of Polyvinyl Chloride-Wood Fibres. III: Effect of Silane as Coupling Agent”, Journal of Vinyl Technology, Vol. 12, No. 3, September 1990, pp. 142-153 disclose modified polymer (other references disclosed modified fibre) in highly plasticized thermoplastic composites. Additionally, Chahyadi et al., “Wood Flour/Polypropylene Composites: Influence of Maleated Polypropylene and Process and Composition Variables on Mechanical Properties”, International Journal Polymeric Materials, Volume 15, 1991, pp. 21-44 discuss polypropylene composites having polymer backbone modified with maleic anhydride.
Consequently much importance is placed on the coupling of wood or natural fibres with polymer matrices in composites and in particular in thermoplastic-wood composites. The present invention offers the possibility of introducing compatiblisers, which can also function as binders and/or as components of the plastic matrix (depending on loading) by applying the methods of the MDF and related processes.
It should be noted that although many publications refer to wood fibres, in reality they are essentially wood flour or powders, or flakes, or saw-dust etc rather than fibres with a reasonable aspect ratio (10:1 or 20:1 or more) and for example typically have an average length of less than 0.8 mm or 1 mm. For the purposes of the current invention fibres are the products of MDF type processes as described herein and in other publications, or are mechanical or thermomechanical pulping or high temperature mechanical pulping or other similar refining processes or processes incorporating such refining processes as one stage and can, optionally, include chemical or biological pulping stages. Such fibres will have average fibres of ˜1 mm or more, though of course may contain fines or fibres of lower lengths as components. Accordingly, a substantial need exists for improved processes to introduce wood or other natural fibres, for example with an aspect ratio greater than 10:1 or 20:1 or with an average fibre length of, say, 0.8 mm or 0.9 mm or 1 mm or more (as opposed to wood flout or powders or flakes) into plastics in the form of composite products which are sheets, impregnated mats, prepregs, boards, films, laminates or profiles or moulded products or shapes machinery, and, also, optionally for compatibilising formulations or materials for combining thermoplastic polymer(s) and wood or natural fibre(s) and also, again optionally, for introducing functional additives into the fibre-plastic combination.
In other prior art, Sears et al (Sears K. D et al, 2001. Proc. 6th International Conference on Woodfibre Plastics Composites, Forest Products Society, 2001, p 27-34 and U.S. Pat. No. 6,270,883.)) describe the use of fibres with an alpha cellulose content purity>80% indicative of pulps which are usually kraft or chemically pulped and not ordinarily available or manufactured by an MDF type process ie via mechanical or thermomechanical pulping methods such as used in the present invention. These and related methods use liquid immersion or dip-coating or slurries for impregnation of fibres rather than MDF and related processes, which are better suited to low cost continuous manufacturing processes.
Medium density fiberboard (MDF) uses a high temperature thermomechanical pulp fibre to produce large panels for a variety of applications, such as furniture or internal mouldings. MDF takes much of its characteristics from the fact that it uses wood cells (tracheids, vessels, fibres and fibre-tracheids), rather than particles. Below is a description of aspects of one variation of a process for MDF manufacture. A Masonite gun Process, Atmospheric or Pressurised Disk refiner is used and for example, the Asplund defibrator pressurized disk refinement is commonly used in MDF manufacture. The chips are compacted into small plugs which are heated, to soften the wood, and then fed into the defibrator where high temperatures are often applied to defibrate wood. The pulp may pass through a secondary refiner or other stages. The resulting pulp is light, fine, fluffy and fibre walls are largely intact. After defibration fibres may enter the blowline and usually the fibres pass through at high velocity. Wax, often used to improve the moisture resistance of the finished product is sometimes added, and resin is added, usually in the blowline while the fibres are still wet. The blowline feeds into a tube drier further into the process and fibres are dried, for example by hot air heated from a boiler or via heating coils warming the blowline. Agitation of fibres in the blowline helps disperse resin consistently. The fibres exit warm-hot and may be stored in bins for an unspecified length of time, though often less than 30 minutes, and the board making process is usually continuous from here on. The moisture content of the fibres is often ˜12% and considered a dry process.
The blowline mixing process and the use of dry fibres are distinguishing characteristics of MDF. A continuous (usually but not essentially) and consistent mat is formed from the flowing fibre stream (at considerable air velocities) and one way of achieving this is a Pendistor wherein impulses of air act on the fibre as it falls down a shaft to a conveyor or to a vacuum box at the start of the conveyor belt that carries the mat. The oscillatory action on the fibres spreads them uniformly into a mat and they begin their run on the conveyor belt at selected thicknesses. The mats are them pressed, with heat, to certain thicknesses or density profiles.
Due to the commercial scale of these operations and the use of heat to soften the fibres, MDF fibre is a low cost form of wood fibre. Additionally, it has an aspect ratio to allow reinforcing of composites (for example radiata pine approx 2.5 mm×30 μm). In the MDF process a thermosetting resin, typically urea-formaldehyde (UF) or related formaldehyde crosslinking resins, or other resins such as isocyanate resins, is added to the fibre, with the fibre in a wet state, while it is exiting the refiner in the blowline. This gives extremely high surface coverage of the fibre at low resin loadings. The MDF or particle board type processes would not be usually associated with producing a thermoplastic or reformable product or a resin or polymer rich sheet. Ordinarily it produces a sheet product which is fibre-rich and bound with a rigid, cured, thermoset resin which is not readily thermoplastically processable or reprocessable or easily usable or useful in thermoplastic processes.